Core Network and Service Layer Realization

Multimedia Telephony makes use of the IMS functionality in the service layer. In Figure 3.25 one possible realization of Multimedia Telephony is shown. The system presented includes the IMS as outlined in Section 3.5 and information about the functions and interfaces can be found there. The text below highlights some of the functions that are of special interest for Multimedia Telephony and are part of this particular realization of the Multimedia Telephony communication service. The functions of interest are marked in the figure with numbers that correspond to the numbers in the text below.

Telephony Interworking client

Figure 3.25: Realization of Multimedia Telephony from an architectural view.

Telephony Interworking client

Figure 3.25: Realization of Multimedia Telephony from an architectural view.

The application server (1), which here is called the Telephony Application Server (TAS), is a crucial part of the Multimedia Telephony infrastructure. The TAS contains service-specific logic for the Multimedia Telephony communication service. This service-specific logic includes mechanisms that handle setup, teardown and modification of Multimedia Telephony sessions. Logic to handle supplementary services like conferencing and communication diversion is also included. For more information about Multimedia Telephony session handling and supplementary services see Sections 4.4 to 4.7. The TAS also does charging and subscriber handling with the help of the HSS.

The Multimedia Telephony client (2) is another crucial part of the system. The Multimedia Telephony client includes the Multimedia Telephony communication service, potentially other IMS communication services like Presence and the Multimedia Telephony application. The Multimedia Telephony communication service is the part that originates or terminates Multimedia Telephony session signaling and media while the Multimedia Telephony application implements amongst other things the end-user to client interfaces (see Figure 3.26). When producing media streams, the Multimedia Telephony client accesses shared resources like codec and protocol stacks implemented in the mobile terminal. For more information about codecs and protocols used in Multimedia Telephony see Sections 5.1 and 5.2. The Multimedia Telephony client implements media transport processing mechanisms to handle jitter and packet losses and it also handles media adaptation if needed. These mechanisms are presented in Sections 5.3 and 5.4.

Figure 3.26: Terminal architecture for a Multimedia Telephony client on the application level. On top is shown the Multimedia Telephony application, which may utilize different IMS communication services and XDM. A common SIP stack is used for the different IMS communication services and XDM that use XCAP over HTTP.

Figure 3.26: Terminal architecture for a Multimedia Telephony client on the application level. On top is shown the Multimedia Telephony application, which may utilize different IMS communication services and XDM. A common SIP stack is used for the different IMS communication services and XDM that use XCAP over HTTP.

XML Document Management (XDM) and Presence (3) are service enablers that can be used together with Multimedia Telephony to ease communication. XDM helps users to store and manage their contact lists in the network rather than in the mobile terminal as of today (for more information about XDM see Section 8.4.2). Presence is a service enabler that allows a user to subscribe to other users' Presence information. Usually the Presence information shows the availability and willingness of a user to communicate, but other information like geographical position and current mood can be included as part of the Presence information (for more information about the Presence communication service, see Section 8.4.1).

The different media types supported by the involved IMS communication services need different QoS. To help control the QoS, 3GPP has developed the Policy and Charging Control (PCC) architecture. The PCRF (4), which is part of the Policy and Charging Control (PCC) architecture, will most probably be a key component in many Multimedia Telephony realizations. The PCRF controls the media plane by the information provided in the control plane via SIP and SDP (for more information about SDP usage in Multimedia Telephony, see Section 5.2.5) and by that PCRF helps operators to provide users with radio bearers that have suitable QoS for the media types they intend to use. PCRF can also help operators to block users who do not have valid subscriptions. The PCRF is an integral part of the system when using network initiated QoS, which is a new feature in 3GPP release 7. For more information about network initiated QoS see Sections 4.3.1.1 and 4.3.2.1. The GGSN (4) is the end-point of the UMTS bearer service, which means that the end-points for the QoS negotiation are the GGSN and the mobile terminal. Therefore, in the PCC architecture the GGSN acts as the policy enforcement point enforcing policies regarding QoS for Multimedia Telephony and other IMS communication services.

CS interworking is needed for fast uptake of Multimedia Telephony. The MGCF and the IMS-MGW (5) provide the functionality of translating CS signaling to/from SIP and perform media conversion. For more information about IMS interworking with CS networks and media considerations of the interworking, see Sections 4.8 and 5.7.

A 3GPP compliant mobile terminal is connected to IMS via either GERAN or UTRAN (6). Both access methods can provide a number of different radio bearer realizations that can be used for Multimedia Telephony. However, UMTS is the most likely candidate for early introduction of Multimedia Telephony. Therefore, in Section 3.7.2 one possible UMTS radio bearer realization for Multimedia Telephony is outlined. The realization is further described, in more detail, in Section 5.6. This radio bearer realization was used for the performance evaluations for Multimedia Telephony in Chapter 7.

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